American third seed Venus Williams moves into fourth round of women's event in Miami

(CNN) -- Defending champion Andy Murray crashed to a shock defeat by American Mardy Fish in his opening match at the Sony Ericsson Open in Miami on Saturday.

The 28-year-old Fish, who is 98 places below Murray at 101 in the world tennis rankings, triumphed 6-4 6-4 to delight a supportive home crowd and set up a third-round clash with Spanish 29th seed Feliciano Lopez.

Australian Open finalist Murray continued a poor run of form which has seen him lose to Robin Soderling in the quarterfinals at Indian Wells and to world No. 39 Janko Tipsarevic of Serbia in his second match in Dubai.

Britain's top player followed world No. 2 Novak Djokovic in making an early exit after the Serbian was shocked by unseeded Belgian Olivier Rochus on Friday.

It's a great win. I certainly had to serve well, and I did when I needed to
--Mardy Fish

Later on Saturday, world No. 1 Roger Federer begins his bid for a record-equaling 17th Masters 1000 tournament victory against Ecuador's Nicolas Lapentti, who made it into the main draw as a lucky loser in qualifying.

Fish fired 10 aces and won 82 percent of his first serves to clinch his first win against Murray since 2005, ending a run of three defeats.

"He's a Grand Slam finalist this year already and his results speak for themselves," Fish told the ATP Tour's Web site. "It's a great win. I certainly had to serve well, and I did when I needed to.

"It got me out of some jams there late in the match. I had, I think, three break-points against, and hit aces on every one of them or unreturnables on every one of them."

Murray had made a strong start, leading 3-1 and 40-0 in the first set before his game fell apart.

"Mardy served well when he needed to, especially the second set. But I just wasn't very good today, and I'm gonna need to get a lot better," he told the ATP Web site.

"If you leave the ball short in the middle of the court, it's very easy for guys to attack and be aggressive. You don't get away with playing that way against guys that can serve well, that are difficult to break and play very aggressive on the return."

Fifth seed Soderling moved into the third round with a 7-6 (7-5) 6-0 win over Australia's Peter Luczak. The Swede will next play Germany's Philip Petzschner, who upset 31st seed Tipsarevic 6-4 6-0.

In the women's tournament, American third seed Venus Williams cruised into the fourth round with a 6-1 6-4 romp over Italy's Roberta Vinci.

The three-time champion, who is unbeaten in 12 matches after winning two successive tournaments, will next face either Russian 16th seed Nadia Petrova or Slovakia's Daniela Hantuchova.

Polish sixth seed Agnieszka Radwanska defeated former world No. 1 Ana Ivanovic 7-5 7-5 to set up a clash with Kazakhstan's Yaroslava Shvedova.